Stinson’s Venerable “Flying Jeep”
Copyright 2021 by James H. Gray
The Stinson L-5 Sentinel is a two-place liaison airplane that was designed in 1941 shortly before America’s entry into World War II. Primarily flown by the U.S. Army Air Forces, it was also operated in lesser numbers by the Army Ground Forces, Marine Corps, and British Royal Air Force. It mainly served in Europe, the China-Burma-India theater, New Guinea, the Philippines, and the Pacific island campaigns. It also saw limited service in Alaska, South America, Central America, and the Caribbean. The L-5 was used extensively within the continental United States too, of course.
The unarmed L-5, or “Flying Jeep” as it was informally christened by Stinson, was designed to provide courier service, officer transport, and artillery adjustment. Once it entered service it was also used for route reconnaissance, aerial photography, medical evacuation, search & rescue, and delivering supplies. After air superiority was established it sometimes performed those missions behind enemy lines. The L-5 was also occasionally used for controlling troop movements, checking camouflage, dropping leaflets, laying telephone wire, and directing fighter bombers on “horsefly” close air support missions. In a few isolated cases, it made aerial attacks using rockets, bazookas, hand grenades, and other small explosives. Its utility on the battlefield was only limited by the willingness of using organizations to attempt new things.
In the United States, L-5’s were used for advanced liaison pilot training and field exercises with army ground forces. Sentinels also aided in training radio and radar operators, anti-aircraft gunners, aerial photographers, and air-evacuation personnel. The experimental development of new equipment and tactics was another common role for the versatile plane. The L-5 was not used for primary training, however. That was a job delegated to the lower horsepower aircraft such as the L-2, L-3, and L-4 instead.
Unlike most other liaison types that were civilian airplanes purchased more or less “off the shelf” and used for primary flight training and artillery spotting, the L-5 was purpose-built and designed to meet stringent Army-Navy Engineering Handbook standards instead of the less rigorous Civil Aeronautics Authority (CAA) requirements. The only other purpose-built American liaison plane that served during World War II was the O-49 / L-1 Vigilant, also manufactured by Stinson. The Sentinel was designed as a cheaper alternative to that much larger, complex and more costly plane.
Stinson O-49 / L-1 Vigilant designed in 1938
The roots of the L-5 can be traced to the pre-war Model 105 (HW-75), and the follow-on Model 10 that was evaluated by the Air Corps in 1940 under the designation YO-54, but it should not be thought of as a modified version of those airplanes. The first tandem-seat prototype, developed in 1940, was a modified 100 hp version of the Model 10, called the Model 75B. Re-designated as the Model 75C with a 125 hp six cylinder engine, it looked a lot like the later Model 76 (L-5) but the similarity was only skin deep. Although the Model 75C handled beautifully, it was rejected by the military on the grounds that it was under-powered, under-flapped, and too lightly constructed.
The first prototype, designated Stinson Model 75B, was first flown in June 1940
The second prototype, designed in 1941 and known as the Model 76, shared the airfoil, slotted flaps, fixed wing slots, and cantilevered main landing gear used on the commercial Stinson models and experimental Model 75B / 75C liaison plane, but those parts were enlarged and strengthened. The rest of the airframe was a clean-sheet design powered by a new six-cylinder Lycoming engine that was developed for the Model 76. Structurally, the new plane was in a league of its own and able to withstand more than twice the g-loads that its predecessors (or liaison planes from competing manufacturers) could handle. None of its components were interchangeable with the earlier Stinson models except for basic hardware and a few flight instruments.
Designed by Athanas P. Fontaine, Chief Engineer, with substantial help from aerodynamics expert Peter Altman, Dean of the Aeronautics Department at the University of Detroit, the Model 76 was test flown on June 28, 1941. After minor changes, it underwent accelerated service testing with the Army Air Forces in August and September. Garnering a tentative manufacturing contract under the designation O-62 just days before the Japanese surprise attack on Pearl Harbor, the first production models did not begin rolling off the assembly line in Wayne, Michigan until a year later. The long delay was due to an aluminum shortage that necessitated a redesign of the wings and empennage using plywood and spruce as a substitute. The change also required re-training workers and re-tooling the assembly line to accommodate the changes. By the time the first production examples rolled off the assembly line in November 1942, they had been re-designated as L-5’s.
The second prototype, designated Model 76, was first flown in June 1941 and became the L-5.
All L-5 production models had wooden wings, empennage, and doors, and the airframe was covered in linen finished with butyrate dope. The fuselage was constructed using heavy-wall chrome-moly steel tubing also covered in linen. Early wing struts were steel but later ones, beginning in 1944, were made from aluminum. Up until that time the factory was forced to minimize the use of that material which was in short supply. It was used in the rudder, elevator, ailerons, “boot cowl”, engine cowl, tail cone, wing slats, window frames, fuselage fairings, and gap seals. Lightweight phenolic material was employed for non-structural items such as the instrument panel. The upper surfaces of the finished planes were painted two-tone non-specular green camouflage and all lower surfaces were finished in medium gray.
The 1942 redesign of the wings and tail delayed production for almost eleven months but it released strategic materials for use on other aircraft being manufactured by Vultee. Clever engineering also strengthened those components to surpass a military requirement for the airframe to withstand repeated 6g pullouts from steep dives — far more than any of the civilian planes adapted for liaison work could sustain. The wooden wings withstood more than +12g without failure during static testing at Wright Field. That topped the design strength of the Douglas SBD dive bomber wings and is comparable to the stress limits of modern aerobatic airplanes such as the Pitts S-2 biplane.
The Sentinel was stout in every aspect but the penalty was increased weight that led to somewhat greater takeoff and landing distances and a lower climb rate compared to the Model 76 prototype that had impressed the military evaluation boards in 1941. However, the trade-off in performance for greater strength gave the L-5 remarkable agility. Pilots loved its ability to perform evasive maneuvers that the comparatively frail L-4 Cub and other liaison aircraft couldn’t manage without risking structural failure. It was also more survivable in a crash due to the heavy construction, particularly after 4-point restraint harnesses were introduced.
The landing gear of the L-5 was quite robust, being designed to withstand a 10g vertical drop. The tapered tubular main landing gear legs were damped with long-stroke oleo shock absorbers and the steerable tailwheel assembly was supported on a trailing arm damped with an oleo strut incorporating a coil-over spring. This gave the Sentinel the ability to absorb bone-jarring rough-field landings without back-breaking results. Unfortunately, its Achilles Heel was that the main gear legs were not as resistant to sheering forces and gear legs sometimes collapsed after tangling with an unseen gully, drainage ditch, shell hole, or berm. Once damaged in this way, field repairs were difficult so many L-5s were written off after such mishaps.
The six-cylinder Lycoming O-435 engine that powered the L-5 had no such frailties. Producing 185 horsepower (190 in the L-5G), it was designed in 1941 specifically for Stinson’s prototype Model 76. This powerplant became renowned among mechanics for longevity under combat conditions, and with minimal care it routinely flew more than twice as many hours between overhauls as the 65 horsepower four-cylinder Continental engines used on the Piper L-4 Cub, giving the L-5 a substantially higher in-service ratio. The power of the O-435 gave the Sentinel other advantages over the Cub such as the ability to operate from high-elevation mountain airstrips while still carrying an observer and radio equipment.
Reliability was also a key attribute, and pilots implicitly trusted the O-435 to carry them over long stretches of water, jungle, and mountainous terrain. If an adequate supply of fuel was carried, the O-435 rarely missed a beat. In one instance, using auxiliary fuel tanks allowed a squadron of L-5B’s to fly 750 miles over water from the Philippines to Okinawa, an amazing feat for a group of so-called “puddle-jumpers”. That speaks volumes about the confidence placed in the “flat six” Lycoming engine. Its biggest enemy was carburetor ice which is something all normally aspirated aircraft piston engines are susceptible to under the right atmospheric conditions. Enough L-5’s suffered forced landings due to carburetor icing that carburetor heat was required for all operations except takeoff when the temperature was below 60 degrees Fahrenheit. A bigger complaint from pilots, however, was the lack of cockpit heat which wasn’t one of the military design requirements.
PRODUCTION and VARIANTS
Exactly 3,590 Sentinels were delivered under contract by the end of World War II and another 35 to 40 more planes were finished and sold on the civilian market in 1946 and 1947. Altogether, six sub-models were manufactured in two basic styles. The original configuration, shown above, featured a wrap-around “greenhouse” that was designed with artillery spotting in mind. Exactly 1,813 of these “observer” type airplanes were produced under the Army designation L-5, Navy designation OY-1 and RAF name Sentinel I .
A substantial factory modification of the fuselage was implemented in early 1944, featuring a large two-piece rear door allowing a stretcher-bound patient or 250 pounds of cargo to be quickly loaded. The ambulance / cargo version pictured below was produced in five variations including 712 L-5B’s, 200 L-5C’s, 500 L-5E’s, 250 L-5E-1’s, and 115 L-5G’s. The expanded utility provided by the cavernous, box-like fuselage was particularly useful in the campaigns fought on Okinawa, Iwo Jima, in the Philippine Islands and in the Chine-Burma-India theater. However, the more restricted view toward the rear made it less desirable for artillery spotting and more vulnerable to hostile aircraft approaching from below and to the rear. To mitigate that possibility in some combat zones, flying at tree-top level was common practice.
The first production L-5B, christened Sentinel II by the British, rolled off the assembly line in June 1944. The C-model came along in January 1945 and included a mount for a K-20 aerial camera (cameras were not delivered with the planes). The E-model premiered in February of 1945 and saw the addition of manually drooping ailerons for improved low-speed handling and slightly shorter takeoffs and landings. The L-5E-1 received larger wheels and tires starting in late May that aided in operations on soft ground. Appearing in July 1945, the L-5G featured a 24-volt electrical system that standardized it with most other military planes. All the previous models had been fitted with 12-volt electrical systems. The G-model also had an improved carburetor and air induction system that increased the engine output from 185 to 190 hp while reducing fuel consumption by an average 1.5 gallons per hour. None of the L-5G’s (called OY-2’s by the Navy) entered service before the war ended, but six years later a number of them served in the Korean conflict.
By modern civil lightplane standards, the performance of the L-5 is considered average and unimpressive, but during World War II it was deemed very respectable. Nominally capable of operating from a 600 foot long dry sod runway at maximum gross weight, when less heavily laden it could easily cope with half that distance in competent hands. During testing at Fort Sill in August 1941, the prototype was consistently able to take off and land within 200 feet with just the pilot aboard. However, in normal use the heavier production aircraft faced with a takeoff over a 50-foot obstacle from a rough, unpaved runway needed 800 to 1,000 feet to clear an obstacle. This was considered the minimum safe length for daily operations although strips half that length were sometimes used. On soft ground or in deep grass, the takeoff distance could exceed 1,500 feet to clear a 50-foot obstacle, so in that instance, the lightweight L-4 Cub was often superior for getting airborne more quickly. Due to its lower mass the Cub was also easier to stop when landing on icy or muddy short strips where braking was poor to nil. Overall, however, the L-5 was vastly superior to the the Cub and other “grasshoppers”.
The L-5 held some distinct advantages. It could climb at a sustained high angle of attack and virtually hang on it’s 7-foot propeller in slow flight without stalling. With its big slotted flaps lowered 45 degrees it could drop over an obstacle into a short strip in an impressively steep power approach without having to side-slip. On a dry, packed surface, its hydraulic brakes allowed the L-5 to stop in as little as 150 feet with no headwind. These are some of the capabilities that gave the L-5 its legendary but somewhat over-rated notoriety as a STOL airplane. While its short field performance couldn’t compare to the utterly amazing Stinson O-49 / L-1 that it replaced (the production of which was ended in 1942 and later much regretted by many), the Sentinel was deemed an acceptable alternative to the much larger, far more expensive, and more complicated no-compromise Vigilant.
All things considered, the “Flying Jeep” exceeded expectations in its appointed role and its reputation as a reliable workhorse was well deserved.
© 2021 James H. Gray / SOPA
[If you quote from this article please give appropriate acknowledgement of the SOPA website and author .]